I had this too today with the new patch, 29/7 (Blaze!). I'm on a desktop iMac. I deleted the Blizzard launcher, reinstalled it, tried to update SCII and HoTs and the launcher itself had an update. I kept getting that same message "waiting for another installation or update." I also couldn't quit the launcher, had to Force Quit several times while doing all these steps.

I tried scan and repair. Wouldn't complete the scan and repair.

I shut the launcher down for a while, re-opened the launcher and it updated the launcher (supposedly) and SCII but HoTs just won't complete. So, I deleted then reinstalled HoTS itself.

Now, I keep getting the Error BLZBNTAGT00000BB8 and I tried everything listed and it still won't complete updating. I keep getting the same error and when I try to run scan and repair, I get "Whoops! Looks like something broke. Give it another shot." and the above Error again.